During drilling operations for oil and gas, items, such as drilling pipe, drilling collars, casings, logging tools, testing tools, wire line, hand tools, tong parts, slip segments, bit cones, chains, expensive measurement while drilling (MWD) tools, and directional drilling packages, as examples, are often dropped into or forgotten in a wellbore. Once a component is lost, it is properly referred to as a “fish.” Therefore, mechanical devices used to aid in the recovery of equipment lost down hole are called fishing tools.
Generally, items introduced into a wellbore are accurately measured and sketched, so that appropriate fishing tools can be selected if the item must be fished out of the hole and diagnostic tools are required. Optical digital cameras may be used for wells which are optically clean. However, wells do not often contain clean fluids and must be flushed with water before images can be taken, a process which may several days, thereby increasing the cost of the fishing operation. Therefore, industry practice is to leave mud in the pipe during fishing operations. Some cameras use lenses to which surfactants have been applied to the surface to allow imaging through hydrocarbons that are transparent. However, there are currently no imaging devices available that allow imaging through mud (oil or water based).
In such situations, an impression block made of soft metal, usually lead, is dropped onto the fish so that upon subsequent inspection, generally on the surface, a custom tool may be designed to facilitate attachment to and retrieval of the fish.
Although fishing occurs frequently, most fishing tasks are relatively simple and the lost time is confined to that of operating the fishing tool and retrieving the fish. However, the impression block technique does not always provide adequate information, and some jobs become costly and time consuming, resulting on occasion in the loss of the hole. It has been estimated that fishing operations account for 25% of drilling costs worldwide.